Vibrational frequency-shifts of H2O caused by complex formation with a molecular cation: a density functional study

Abstract

Structure and vibrational frequencies of the benzene–water complex cation [BzH₂O]⁺ have been calculated by means of density functional theory (B3LYP calculation). A planar structure with a C_s symmetry, in which all heavy atoms are located on a molecular plane, was obtained as the most stable form of the [BzH₂O]⁺ cation. Hydrogen atoms of the water molecule are located above and below the molecular plane. From the present calculations, it was predicted that vibrational frequencies of symmetric and asymmetric O–H stretching modes of H₂O (ν₁ and ν₃ modes, respectively) are red-shifted from those of a free H₂O by the formation of a complex, whereas the H–O–H bending mode (ν₂ mode) is blue-shifted. Infrared intensities of the three modes of H₂O were significantly increased by the complex formation. Similar features were obtained for the ethylene–H₂O complex cation [C₂H₄–H₂O]⁺. The origin of the frequency-shifts is discussed on the basis of theoretical results. The hydrogen-hyperfine coupling constants of the complex cations were also predicted theoretically.